The present invention relates to method and apparatus using suppression of eluents for the analysis of anions or cations in ion chromatography.
Ion chromatography is a known technique for the analysis of ions which typically includes a chromatographic separation stage using an eluent containing an electrolyte, and an eluent suppression stage, followed by detection, typically by an electrical conductivity detector. In the chromatographic separation stage, ions of an injected sample are eluted through a separation column using an electrolyte as the eluent. In the suppression stage, electrical conductivity of the electrolyte is suppressed but not that of the separated ions so that the latter may be determined by a conductivity cell. This technique is described in detail in U.S. Pat. Nos. 3,897,213, 3,920,397, 3,925,019 and 3,926,559.
Suppression or stripping of the electrolyte is described in the above prior art references by an ion exchange resin bed commonly referred to as a packed bed suppressor (PBS). The PBS requires periodic regeneration by flushing with an acid or base solution.
Another approach to regeneration of a PBS is disclosed in U.S. Pat. No. 5,773,115. Ion chromatography is performed by chromatographic separation, chemical suppression in a packed bed and detection. Thereafter, an electrical potential is passed through the packed bed suppressor while flowing an aqueous stream through it to electrolyze water in the stream and thereby create hydronium or hydroxide ions to regenerate the ion exchange resin. The packed bed suppressor has electrodes embedded in the resin for electrochemical regenerant. A second ion exchange resin bed is disclosed with suitable valving to pass liquid streams through the system. In one alternative, a second sample in an eluent stream is chromatographically separated. The eluent and separated second sample flow through a second packed bed suppressor and to a detector. The effluent then flows through the first packed bed suppressor, forming the aqueous liquid stream required for regeneration and an electrical potential is applied for regeneration. The second suppressor may be similarly regenerated by flowing the detector effluent of the first sample through it and applying an electrical potential.
Another form of suppressor known as a “membrane suppressor” is described in U.S. Pat. No. 4,999,098. In this apparatus, the suppressor includes at least one regenerant compartment and one chromatographic effluent compartment separated by an ion exchange membrane sheet. The sheet allows transmembrane passage of ions of the same charge as its exchangeable ions. Ion exchange screens are used in the regenerant and effluent compartments. Flow from the effluent compartment is directed to a detector, such as an electrical conductivity detector, for detecting the resolved ionic species. The screens provide ion exchange sites and serve to provide site to site transfer paths across the effluent flow channel so that suppression capacity is no longer limited by diffusion of ions in the bulk solution to the membrane. A sandwich suppressor is also disclosed including a second membrane sheet opposite to the first membrane sheet and defining a second regenerant compartment. Spaced electrodes are disclosed in communication with both regenerant and chambers along the length of the suppressor. By applying an electrical potential across the electrodes, there is an increase in the suppression capacity of the device. The patent discloses a typical regenerant solution (acid or base) flowing in the regenerant flow channels and supplied from a regenerant delivery source. In a typical anion analysis system, sodium hydroxide is the electrolyte developing reagent and sulfuric acid is the regenerant. The patent also discloses the use of water to replace the regenerant solution in the electrodialytic mode.
Another membrane suppressor is described in U.S. Pat. No. 5,248,426. A direct current power controller generates an electric field across two platinum electrodes to electrolyze water in the regenerant channels. Functionalized ion-exchange screens are present in the regenerant chambers to facilitate electric current passage with permselective ion-exchange membrane defining the chromatography eluent chamber, as in the '098 patent. After detection, the chromatography effluent is recycled through the suppressor to form a flowing sump for electrolyte ion as well as providing the water for the electrolysis generating acid or base for suppression.
A different membrane suppressor is disclosed in EPA Publication WO 99/44054. The suppressor is of the membrane suppressor type even though it includes a flow-through suppressor bed of ion exchange resin. The bed has a liquid sample inlet and an outlet section, a first electrode in an electrode chamber is adjacent to the suppressor inlet section. A barrier separates the suppressor bed from the electrode chamber, preventing significant liquid flow but permitting transport of ions. A second electrode is in electrical communication with said resin bed outlet section. A recycle conduit provides fluid communication between the suppressor outlet and the electrode inlet. In one embodiment of the disclosed method for anion analysis, effluent from a chromatography column is suppressed in cation exchange resin in the suppressor. The effluent from the suppressor flows past a detector and is recycled to the electrode chamber including a cathode. An electrical potential is applied between the cathode and an anode in electrical communication with the suppressor bed. Water is electrolyzed at the anode to generate hydronium ions to cause cations on the cation exchange resin to electromigrate toward the barrier and to be transported across said barrier toward the cathode while water in the cathode chamber is electrolyzed to generate hydroxide ions which combine with the transported cations to form cation hydroxide in the electrode chamber.